This invention relates to an optical write device for a color video printer, a color printer or the like, and more particularly to a color print head incorporating a fluorescent luminous tube therein.
A fluorescent luminous tube which is adapted to select electrons emitted from filamentary cathodes through control electrodes to impinge the electrons on anodes, to thereby provide desired luminescence may be used for a print head for a color printer as exemplified in Japanese Patent Application Laid Open Publication No. 258920/1989.
The conventional fluorescent luminous tube, as taught in the Japanese publication, is so constructed that three strip-like anodes which extend in a direction parallel to a main scanning direction are juxtaposed to each other in a sub-scanning direction on a substrate. The anodes are provided with phosphors R, G and B different in luminous color from each other, respectively. Above the anode are arranged a plurality of grids formed with slits extending in the sub-scanning direction and divided in the main scanning direction. Picture cells are defined by cooperation of a width of the anodes and a width of the slits of the grids and selection of any desired picture cells is carried out, for example, by scanning the anodes and feeding the grids with a print signal. Picture cells respectively emitting lights of R, G and B luminous colors which are arranged in the sub-scanning direction are optionally selected to converge the lights.
Thus, it should be noted that the conventional color print head uses the phosphors R, G and B different in luminous color. In general, a phosphor is deteriorated in luminance with a lapse of lighting time. However, a variation in residual ratio of luminance of a phosphor depends on the type of a phosphor. For example, luminance of a phosphor of a blue luminous color is deteriorated most rapidly. Therefore, the conventional color print head is encountered with a problem that a balance of luminance is varied with a lapse of lighting time, resulting in failing to permit it to exhibit color display required or desired.
A phosphor which has been conventionally used for such a purpose is a sulfide phosphor of which a matrix contains sulfur. Thus, when the sulfur phosphor is excited due to impinge of electrons thereon, it produces sulfide gas such as S, SO, SO.sub.2 or the like, resulting in being decomposed and scattered. The decomposed and scattered phosphor then adheres to an oxide layer of the filamentary cathodes to poison them, to thereby deteriorate an electron emission capability of the cathodes.
The above-described sulfide tends to adhere to the phosphor. Such adhesion causes a luminous efficiency of the phosphor to be varied, leading to unstableness of luminance of the phosphor.
Further, a conventional phosphor for color display has a conductive material such as In.sub.2 O.sub.3, SnO.sub.2, ZnO or the like mixedly incorporated therein in an amount of several %. However, it is highly troublesome or difficult to homogeneously incorporate the conductive material in the phosphor. Also, the fact that dimensions of each of luminous dots are as micro as 60 .mu.m.times.60 .mu.m causes uniform or homogeneous mixing of the conductive material to be substantially impossible. Thus, the conventional phosphor fails to permit the luminous dots to emit light at uniform luminance.
Moreover, the conventional color print head generally requires to superposedly irradiate light of the luminous dots R, G and B on one location at the fully same configuration. In this regard, in the conventional color print head, configuration of the luminous dots is restricted by a width of the slits of the grids and a width of the phosphors deposited on the strip-like anodes. Unfortunately, the phosphors are formed by printing or electrodeposition, so that it is substantially impossible to render dimensions of the phosphors in a width direction of the anodes uniform with precision as high as several microns. This results in a configuration of the luminous dots different in luminous color being non-uniform, to thereby cause bleeding, color shift and the like, leading to deterioration of printing quality of the color printer.
In addition, reproduction of color printing by the conventional color print head is carried out by resolving one dot into R, G and B dots and permitting the R, G and B dots arranged in the sub-scanning direction to emit lights while superposedly converging the lights. Therefore, deterioration in positional accuracy of the R, G and B dots causes color shift, bleeding and the like, leading to further deterioration of printing quality of the color printer. In the conventional color print head, a position of the luminous dots is determined by the slits of the grids, therefore, a failure in accurate positioning of the grids causes positional deviation of the luminous dots.